1. Field of the Invention
The present invention relates generally to an injection molded thermoplastic ophthalmic lens. More particularly, the present invention is directed to an improved method for manufacturing an injection molded thermoplastic ophthalmic lens having an encapsulated light polarizing element.
2. Description of Related Art
There are numerous advantages associated with using highly transparent, optical quality plastic in place of glass when forming ophthalmic lenses. Plastic lenses are lighter, tougher, and less likely to fracture than glass lenses. Further, plastic lenses are processed quite differently than those of glass. The elevated temperatures necessary for the manufacture of a glass ophthalmic grade lens are not required to process a plastic ophthalmic grade lens. The secondary processing necessary for the addition of a prescription to the thermoplastic ophthalmic lens are greatly reduced due to the reduction in the need for the extensive grinding and polishing operations required to be performed on glass lenses. One such glass lens is disclosed in U.S. Pat. No. 3,051,054 issued to Crandon on Aug. 28, 1962 where a glass element is utilized in construction of a dichroic polarized wafer assembly. As previously stated, the prohibitive cost of the processing of both the glass/wafer composite structure and the subsequent secondary processing difficulties makes it undesirable to manufacture such a glass lens. Also, the exposure of the polarized portion of the glass lens to abrasion and weathering reduces the expected life of the glass lens element.
Plastic, synthetic resin ophthalmic lenses have historically been manufactured through the use of an injection molding machine to inject a synthetic, thermoplastic, optically clear, resin material into a mold that has anterior and posterior optically correct, polished insert surfaces. Within such mold, a light polarizing shaped wafer is positioned to allow the injected thermoplastic material to flow directly adjacent to the light polarizing shaped wafer""s anterior and posterior surfaces. This typically involves the injection of a thermoset monomeric material immediately adjacent to each side of a light polarizing element secured in the mold, such as disclosed by U.S. Pat. No. 3,833,289, issued to Schuler on Sep. 3, 1974. In such a method, a light polarizing element having projections about its periphery to provide a predetermined distance between at least one face of the light polarizing element and a complimentary mold component is positioned into a resilient mold component. This same result can be accomplished by providing a shim of a predetermined thickness between the mold component and the polarized wafer.
It has been found that utilizing this method of molding ophthalmic lens structures, under certain circumstances, causes free monomer or other uncatylyzed constituents of these components to migrate from the monomeric materials that make up these structures into the light polarizing element. Such migration into the light polarizing element which forms an integral component of the ultimately formed lens thereby dexterously affects the light polarizing element by bleaching some of the dyes comprising the element, where the migration of the monomer into the light polarizing element can cause it to deform and buckle. In addition, plastisizer components of the spacing structures were sometimes found to migrate therefrom into the polymerizing monomer which causes the formation of a xe2x80x9csoftenedxe2x80x9d polymeric material generally containing striations and haze.
There is clearly a need for an improved method of forming an injection molded thermoplastic ophthalmic lens which obviates the migration of catalysts, monomeric materials, and plastisizers into the light polarizing element. Further, there is a need for an improved method of forming an injection molded thermoplastic ophthalmic lens which eliminates the necessity for adjunct support and spacing mechanisms for a light polarizing element within the mold which can lead to such migrations.
The foregoing shortcomings and disadvantages of the prior art are alleviated by the present invention which provides an improved method of manufacturing a light polarizing, laminated, ophthalmic lens made up of thermoplastic synthetic resin. The improved method of the present invention provides for precision placement of a light polarizing element within an injection mold prior to and during the molding process along with a unique injection/compression molding procedure following injection of the thermoplastic material.
The method for manufacturing a synthetic thermoplastic resin ophthalmic lens according to the present invention comprises the injection of a thermoplastic resin material immediately adjacent to each side of a light polarizing element or wafer secured and oriented within a mold cavity. The wafer is formed having spaced-apart projections extending outwardly from its periphery which are positionable into registration notches in the mold cavity in order to maintain a desired position of the wafer within the mold during the lens formation process. A melted thermoplastic material is then injected into the mold cavity at low pressure about anterior and posterior surfaces of the wafer. Once a desired amount of thermoplastic material has been injected into the mold cavity, a compression procedure is implemented where at least one of the mold surfaces is moved in the direction of the other mold surface to compress the contents of the mold cavity. The mold cavity is compressed to a predetermined position to achieve a desired thickness and shape of the lens, where uniform compression is exerted over the entire surface of the lens during the compression procedure. As a result of the uniform compression, all stresses are uniformly distributed over the lens surface to significantly negate stress-inducted birefringence.
The combination of precisely positioning the wafer within the mold, injecting optical quality thermoplastic material under low pressure into the mold, and uniform post-injection compression of the mold cavity area provides a thermoplastic ophthalmic lens free of process constituent migration problems associated with the wafer registration in the mold. Further, the formed thermoplastic ophthalmic lens exhibits a low occurrence of defects due to the positioning and anchoring of the wafer, thus minimizing birefringence. The improved method of forming a thermoplastic ophthalmic lens of the present invention additionally ensures adequate adhesion between the components of the composite structure and provides a finished lens having uncompromised optical clarity.